There is no single way to do this strum. Can be:
Where P is the thumb, and M the fingers of his right hand. The arrows indicate that the strings are pressed from the 6th to the 1st, with the thumb the most serious (eg. 6th, 5th, 4th and 3rd), and with the other fingers the strings more acute (eg. 4th, 3rd, 2nd and 1st). With the yolk the thumb you must make a strum to quickly over the strings, to ring almost all at once, like a fast arpeggio. With the remaining fingers (the pinky is not used), you must make a strum with fingernails on 3 or 4 strings simultaneously, producing a snap sound , which contrasts with the soft sound of the strings pulsed with the yolk the thumb. The palm of the hand is used to stop the sound, resting very soft over the strings so that is not too sudden.
To make things easier I put an animation with sound with the movement of the hand so that you can imitate it. With what you already know, should not take long to learn to do so with some ease.